Motors and actuators are used in myriad applications. The system requirements for many of these applications are requiring ever-wider dynamic output torque ranges, while maintaining sufficient resolution. For example application is for missile guidance systems, where one or more multi-phase electric motors or actuators are used to control a thruster nozzle, which controls where the missile goes. More specifically, the motors supply output torques to hold the nozzle in place or to reposition the nozzle, in order to hold or change, respectively, the course of the missile. Over the duration of a missile flight, the motors or actuators may need to produce output torques that dynamically vary over a relatively wide range.
The agility of a missile, which is defined as its ability to avoid obstacles and make last minute adjustments for target acquisition, is controlled by its dynamic, high torque capabilities. High resolution torque requirements are necessary for stability, fine pointing, and compensation for minor disturbances due to weather. Thus, the wider the torque range, the more agile the missile; however, the resolution needs to be maintained to control the stability.
A motor produces output torque by having current driven through its windings. As is generally known, the torque produced by each motor winding at any given time is the product of the current supplied to the winding and the torque constant of the winding for the winding position at that time. The sum of each of the individual winding torques is the motor output torque.
Because motor current is an analog parameter, many heritage motor drive control systems were analog-based systems. Even though digital electronics can be used to control analog parameters such as motor current, digital motor drive control systems, at least until recently, were not significantly utilized. One reason for this is because processors did not possess sufficiently fast processing speed capability to handle multiple motor winding current loops. Also, previously known digital motor drive control systems paid a relatively large power and/or weight penalty as compared to the analog-based systems that performed the same function. More recent digital motor drive control systems have the speed and processing capability to handle multiple current loops. These digital systems also have the processing capability to allow multiple signals (acceleration, speed, rate and position) to be derived from a single detector, thereby reducing the number of mechanical components. This latter factor helps to balance, or perhaps tip, the scales in the area of weight.
It was previously noted that, at least for some applications, it is desirable to widen the torque range of a motor, while at least maintaining sufficient resolution. Two methods have been identified to achieve this goal. One of the identified methods is to use two motors scaled to operate over different torque ranges and with their performances meshed together, the other identified method is to reduce the ground floor noise. With regard to the two motor method, including additional motors in a system can adversely impact weight and cost, especially in airborne environments such as missiles and other projectiles.
As to the method of reducing ground floor noise, significant progress has been made. In an analog motor drive control system, resolution is generally defined as the level of ground floor noise. Heritage analog motor drive control systems typically operate off of ±12 VDC or ±15 VDC secondary power sources. Thus, the normal operating range for these systems is in the range of about +10V to −10V. A typical system with a range of +10V to −10V and a ground floor noise of 5 ma has about 12-bits of resolution. The ground floor noise in some analog motor drive control systems has been pushed down to 1 ma, which is equivalent to 14-bit resolution, and some analog motor drive control systems have pushed the ground floor noise down to 0.3 ma, which equivalent to 16-bits of resolution.
With regard to digital motor drive control systems, the simplest of these systems presently have 16-bits of resolution, although some systems have 32-bits of resolution, and others even have 64-bits of resolution. Of course, in these digital motor drive control systems the digital signals have to be converted to analog signals, and analog feedback signals have to be converted to digital signals. However, most digital-to-analog (D/A) converters and analog-to-digital (A/D) converters have resolutions limited to a range of 12-bits to 16-bits. Thus, resolutions greater that 16-bits may not be achievable even in digital motor drive control systems.
Accordingly, it is desirable to provide a method and system for controlling a single multi-phase motor or multi-phase motor driven actuator that increases the control resolution allowing for a wider torque output range. Additionally, it is desirable to provide a method and system for controlling a motor or actuator that increases the control resolution while minimizing torque ripple. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.